1. Field of the Invention
The invention relates to a method for removing an electric crosstalk contribution in a monitoring signal from a monitored electro-mechanical transducer in a device comprising a plurality of electro-mechanical transducers which are driven by actuation signals so as to produce acoustic waves in an acoustic frequency range. More particularly, the invention relates to a method for determining and cancelling electric crosstalk in monitoring signals from transducers of a jetting device such as an ink jet print head, wherein electric signals produced by the transducers are used for monitoring a condition of the jetting device. The invention also relates to a jetting device, more particularly an ink jet print head in which the method is implemented.
2. Description of the Related Art
EP 1 584 474 A1 and EP 2 328 756 B1 describe embodiments of a piezoelectric ink jet print head having a plurality of jetting units for jetting out liquid ink onto a recording medium in order to form a printed image. Each jetting unit has a nozzle connected to a pressure chamber that is filled with liquid ink. The nozzles and, consequently, the jetting devices are arranged at narrow spacings in order to achieve a high spatial resolution of the print head. Each pressure chamber is associated with a piezoelectric transducer, which, when energized by a voltage pulse, deforms in a manner that causes a change in the volume of the pressure chamber. Consequently, an acoustic pressure wave is generated in the liquid ink in the pressure chamber, and this wave propagates to the nozzle, so that an ink droplet is ejected from the nozzle.
Conversely, when a pressure wave is propagating in the liquid in the pressure chamber, this wave will cause a deformation of the transducer, and the transducer will produce an electric signal (voltage and current signal) in response to the deformation. Consequently, as has been taught in the documents cited above, it is possible to detect the acoustic pressure waves in the pressure chambers by monitoring the signals obtained from the transducers.
When a transducer has been actuated, the pressure wave produced by this transducer will gradually decay in the pressure chamber in the course of time. If, for example, an air bubble has been trapped in the pressure chamber or in the nozzle, this will change, in a characteristic way, the pattern in which the pressure wave decays, so that the presence of the air bubble can be detected by monitoring the decay of the pressure wave.
Similarly, the monitoring signals obtained from the transducers may be used for detecting other conditions of the jetting units, e.g. a condition in which a nozzle is partly or completely clogged by contaminants. Examples of other conditions and/or ink properties that may be monitored in this way are the viscosity of the ink and the position of the air/liquid meniscus in the nozzle, which position changes the resonance frequency of the acoustic wave in the pressure chamber.
Since the plurality of transducers of the jetting device form part of a common actuating and monitoring circuitry, and electrical leads of this circuitry are relative closely packed in the device, due to the close packing of the jetting units of the print head, there will inevitably be a certain amount of electric crosstalk between the actuators. Consequently, when one actuator is monitored while the jetting device is operating, the monitoring signal will reflect not only the pressure wave in the jetting unit that is being monitored, but will also include a certain amount of crosstalk from other transducers that have been actuated simultaneously. This may compromise the accuracy in the detection of the condition of the jetting unit.
It is therefore an object of the invention to determine the crosstalk contribution in the monitoring signal, so that this contribution may be eliminated when the monitoring signal is processed and interpreted.